﻿lxxiv 
  PKOCEEDINGS 
  OP 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [May 
  1 
  897 
  

  

  channels 
  before 
  the 
  Lowest 
  Cambrian 
  conglomerates 
  had 
  been 
  

   deposited. 
  1 
  There 
  must 
  have 
  been 
  also 
  an 
  enormous 
  amount 
  of 
  loose 
  

   material 
  on 
  the 
  land 
  to 
  yield 
  the 
  great 
  thickness 
  of 
  Cambrian 
  rocks 
  

   which 
  accumulated 
  as 
  the 
  shore 
  was 
  being 
  depressed. 
  The 
  peno- 
  

   logical 
  descriptions 
  given 
  by 
  Mr. 
  Davies, 
  in 
  the 
  Quarterly 
  Journal 
  

   of 
  this 
  Society, 
  of 
  the 
  volcanic 
  group 
  (Pebidian) 
  as 
  it 
  occurs 
  in 
  

   Pembrokeshire 
  were 
  at 
  the 
  time 
  unusually 
  important, 
  for 
  until 
  

   then 
  no 
  one 
  had 
  suggested 
  that 
  lava-flows 
  and 
  other 
  volcanic 
  rocks 
  

   of 
  so 
  great 
  an 
  age 
  occurred 
  anywhere 
  in 
  Britain, 
  and 
  in 
  his 
  remarks 
  

   on 
  the 
  fragments 
  of 
  rhyolite 
  in 
  the 
  agglomerates 
  he 
  says, 
  p. 
  166, 
  

   vol. 
  xxxiv. 
  (1878) 
  : 
  — 
  ' 
  . 
  . 
  . 
  the 
  whole 
  structure 
  of 
  the 
  rock, 
  in 
  the 
  

   fluidal 
  arrangement 
  of 
  the 
  spherules, 
  etc., 
  is 
  so 
  like 
  that 
  of 
  a 
  rhyolite 
  

   that 
  it 
  is 
  difficult 
  to 
  believe 
  that 
  we 
  have 
  not 
  before 
  us 
  one 
  at 
  least 
  

   of 
  the 
  many 
  interesting 
  varieties 
  afforded 
  by 
  this 
  group. 
  One 
  may 
  

   say 
  with 
  Zirkel, 
  it 
  is 
  a 
  rhyolite 
  petrographically 
  but 
  not 
  geologically/ 
  

   He 
  also 
  calls 
  especial 
  attention, 
  at 
  p. 
  164, 
  to 
  the 
  difference 
  between 
  

   the 
  incipient 
  spherulitic 
  arrangement 
  in 
  the 
  quartz-porphyries 
  con- 
  

   taining 
  the 
  bipyramidal 
  quartz, 
  which 
  occur 
  as 
  dykes 
  in 
  the 
  granitoid 
  

   rocks 
  and 
  in 
  the 
  Pebidian, 
  and 
  the 
  spherulitic 
  structure 
  of 
  the 
  

   rhyolites. 
  He 
  says 
  that 
  the 
  latter 
  often 
  ' 
  weathers 
  cream- 
  white, 
  the 
  

   spherulitic 
  structure 
  when 
  present 
  being 
  then 
  perfectly 
  exhibited 
  

   on 
  the 
  surface. 
  On 
  freshly-fractured 
  surfaces, 
  however, 
  it 
  is 
  not 
  

   to 
  be 
  detected, 
  even 
  with 
  the 
  aid 
  of 
  a 
  lens, 
  and 
  the 
  very 
  large 
  

   spherules, 
  so 
  prominent 
  in 
  the 
  weathered 
  specimens, 
  present 
  but 
  

   faint 
  indications 
  of 
  their 
  presence. 
  Seen 
  in 
  thin 
  sections 
  under 
  

   the 
  microscope, 
  however, 
  the 
  structure 
  is 
  at 
  once 
  recognizable, 
  the 
  

   whole 
  mass 
  appearing 
  to 
  consist 
  of 
  spherules, 
  frequently 
  arranged 
  

   in 
  well-marked 
  bands 
  of 
  varying 
  dimensions, 
  and 
  also 
  confusedly 
  

   grouped 
  without 
  any 
  apparent 
  arrangement.' 
  

  

  The 
  dykes 
  of 
  quartz-porphyry 
  which 
  cut 
  through 
  the 
  granitoid 
  

   rocks 
  and 
  the 
  Lower 
  beds 
  of 
  the 
  Pebidian, 
  but 
  which 
  never 
  have 
  

   been 
  seen 
  to 
  penetrate 
  the 
  Cambrian 
  rocks, 
  are 
  also 
  interesting. 
  

   They 
  are 
  evidently 
  of 
  pre-Cambrian 
  age, 
  and 
  have 
  yielded 
  frag- 
  

   ments 
  to 
  the 
  Cambrian 
  conglomerates. 
  Prof. 
  Judd, 
  who 
  was 
  the 
  

   first 
  to 
  describe 
  specimens 
  of 
  these 
  rocks 
  for 
  me 
  in 
  the 
  year 
  

   1877, 
  says 
  : 
  — 
  ' 
  The 
  rock 
  presents 
  the 
  most 
  remarkable 
  identity 
  of 
  

   character 
  with 
  the 
  gold-bearing 
  quartz-porphyrite 
  of 
  Cstatye, 
  near 
  

   Vorospatak, 
  in 
  Transylvania, 
  which 
  is 
  an 
  eruptive 
  rock 
  of 
  Neogene 
  

   age. 
  It 
  is 
  probably 
  the 
  oldest 
  quartz-porphyrite 
  yet 
  noticed.' 
  2 
  

  

  1 
  See 
  Hicks, 
  'Pre-Cambrian 
  Volcanoes 
  and 
  Glaciers,' 
  Geol. 
  Mag. 
  1880, 
  

   p. 
  488. 
  

   3 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxiii. 
  p. 
  236. 
  

  

  